This invention relates generally to the operation of cryogenic rectification plants, such as cryogenic air separation plants, and more particularly to the provision of refrigeration to a cryogenic rectification plant.
Cryogenic rectification processes often require substantial variation in the amount of refrigeration generated and provided to the plant. The ability to efficiently manipulate refrigeration generation improves the ability of the cryogenic rectification plant to vary liquid production at minimal power expenditure.
Typically the refrigeration for the system is generated by turboexpansion of a process stream which is then passed into the plant. Varying the amount of refrigeration generated in such a system is inefficient because turboexpansion efficiency decreases substantially as operating conditions vary from design conditions, thus limiting the degree to which the refrigeration may be varied. Other attempts to vary the refrigeration generated and provided by a cryogenic rectification plant include the use of variable nozzle turboexpanders, which have a limited operating range, the independent generation of refrigeration, which is capital intensive and expensive, and the use of integrated closed loop refrigeration generation systems, which have operating problems such as leakage.
Accordingly, it is an object of this invention to provide an improved method for providing refrigeration to a cryogenic rectification plant.
It is another object of this invention to provide an improved method for providing refrigeration to a cryogenic rectification plant which enables the facile provision of varying amounts of refrigeration to the cryogenic rectification plant.
The above and other objects, which will become apparent to those skilled in the art upon a reading of this disclosure, are attained by the present invention which is:
A method for providing refrigeration to a cryogenic rectification plant having at least one column comprising:
(A) compressing a working fluid in a recycle compressor to produce pressurized working fluid;
(B) passing a first portion of the pressurized working fluid to a heat exchanger and at least partially condensing said first portion in said heat exchanger to produce liquid working fluid first portion;
(C) cooling a second portion of the pressurized working fluid, turboexpanding the cooled second portion to generate refrigeration, passing the refrigeration bearing second portion to the heat exchanger, and providing refrigeration from the second portion to said first portion to effect the at least partial condensation of the first portion;
(D) passing the liquid working fluid first portion into a column of the cryogenic rectification plant; and
(E) passing the second portion of the working fluid from the heat exchanger to the recycle compressor.
As used herein the term xe2x80x9cfeed airxe2x80x9d means a mixture comprising primarily oxygen, nitrogen and argon, such as ambient air.
As used herein the term xe2x80x9ccolumnxe2x80x9d means a distillation or fractionation column or zone, i.e. a contacting column or zone, wherein liquid and vapor phases are countercurrently contacted to effect separation of a fluid mixture, as for example, by contacting of the vapor and liquid phases on a series of vertically spaced trays or plates mounted within the column and/or on packing elements such as structured or random packing. For a further discussion of distillation columns, see the Chemical Engineer""s Handbook, fifth edition, edited by R. H. Perry and
C. H. Chilton, McGraw-Hill Book Company, New York, Section 13, The Continuous Distillation Process. The term xe2x80x9cdouble columnxe2x80x9d is used to mean a higher pressure column having its upper end in heat exchange relation with the lower end of a lower pressure column.
Vapor and liquid contacting separation processes depend on the difference in vapor pressures for the components. The high vapor pressure (or more volatile or low boiling) component will tend to concentrate in the vapor phase whereas the low vapor pressure (or less volatile or high boiling) component will tend to concentrate in the liquid phase. Partial condensation is the separation process whereby cooling of a vapor mixture can be used to concentrate the volatile component(s) in the vapor phase and thereby the less volatile component(s) in the liquid phase. Rectification, or continuous distillation, is the separation process that combines successive partial vaporizations and condensations as obtained by a countercurrent treatment of the vapor and liquid phases. The countercurrent contacting of the vapor and liquid phases is generally adiabatic and can include integral (stagewise) or differential (continuous) contact between the phases. Separation process arrangements that utilize the principles of rectification to separate mixtures are often interchangeably termed rectification columns, distillation columns, or fractionation columns. Cryogenic rectification is a rectification process carried out at least in part at temperatures at or below 150 degrees Kelvin (K).
As used herein the term xe2x80x9cindirect heat exchangexe2x80x9d means the bringing of two fluids into heat exchange relation without any physical contact or intermixing of the fluids with each other.
As used herein the term xe2x80x9ctop condenserxe2x80x9d means a heat exchange device that generates column downflow liquid from column vapor.
As used herein the term xe2x80x9cbottom reboilerxe2x80x9d means a heat exchange device that generates column upflow vapor from column liquid.
As used herein the terms xe2x80x9cturboexpansionxe2x80x9d and xe2x80x9cturboexpanderxe2x80x9d mean respectively method and apparatus for the flow of high pressure gas through a turbine to reduce the pressure and the temperature of the gas thereby generating refrigeration.
As used herein the term xe2x80x9crecycle compressorxe2x80x9d means an apparatus, such as a mechanical compressor, that increases the pressure of a relatively low pressure working fluid, allowing it to be recirculated.